The goal of this Phase I SBIR is to design, develop, build and test a new prototype of infra red laser thermal stimulator that can be used for differential activation of C and A-delta fibers (nociceptors) in humans. C and A-delta fiber nociceptors transduce and mediate pain of different sources, and are therefore responsible for carrying different types of pain information to the central nervous system. Clinical pain of different underlying pathologies can be differentially mediated by the activation of these two pain fiber types. Thus, the capacity of differential activation of C or A-delta fiber nociceptors in patients may allow for more specific diagnosis, better predictive power in pharmaceutical prescription, and better capacities to follow the clinical progress of the underlying disease. Currently, no tool is available to do the reproducible selective activation of C and A delta fibers in humans and clinically used thermal stimulators rely on electronically controlled contact heaters (Peltier thermodes). These, however, require conductive heating of pain fibers through overlying layers of skin. Similarly, laser stimulators commonly in use for research purposes (CO2 and Thullium) deposit energy entirely on the skin surface, whereas the nociceptors are located deeper in the skin. Thus, for both Peltier thermodes and these lasers, direct heating/activating of nociceptors is impossible, but requires potentially dangerous, high surface temperatures. The laser stimulation system that we propose to develop will use infrared coherent light that, due to its wavelength, allows for direct stimulation of underlying tissue that contains the nociceptor endings. Furthermore, our preliminary evidence suggests that, by altering the parameters of the laser stimulus, we can selectively activate either C or A-delta fibers, potentially allowing for a highly useful clinical tool. Thus, this proposal describes development and experimental work, which should allow for the development of a prototype stimulator that allows for selective activation of C or A-delta fibers in humans